About Astyanax mexicanus (De Filippi, 1853)
The Mexican tetra (scientific name Astyanax mexicanus (De Filippi, 1853)) is also called the blind cave fish, blind cave characin, or blind cave tetra. It is a species of freshwater ray-finned fish that belongs to Acestrorhamphidae, the family of American characins. This species is distributed from the lower Rio Grande, and the Nueces and Pecos Rivers in Texas, through the Central Plateau and into eastern states of Mexico. Mexican tetras mature at a total length of around 12 cm (4.7 in), and have the typical body form of a characin. Surface-dwelling individuals have unremarkable silvery scalation, which is likely an evolutionary adaptation to their natural environment. In contrast, the blind cave-dwelling form of this species has evolved depigmented pinkish-white scales that resemble albino coloration; this is because cave forms live in completely dark subterranean caverns and streams, and have no need for coloration for functions such as mate attraction, camouflage, or UV protection. Similarly, blind cave Mexican tetras have completely lost functional eyes due to their lightless habitat, and only have empty sockets covered by skin in their place. Instead of functional eyes, the blind cave form has highly developed sensory organs along its body, a heightened nervous system, and enhanced senses of smell and touch. It can immediately detect the location of objects or other animals through small changes in surrounding water pressure, a process that is vaguely similar to echolocation, an adaptation also seen in other cave-dwelling and aquatic species such as bats and cetaceans. The blind variant of the Mexican tetra has become steadily more popular among modern aquarists. Like most tetras, A. mexicanus is a peaceful, sociable schooling species. It spends most of its time in midlevel water above the rocky and sandy bottoms of pools and backwaters of creeks and streams. Native to a generally subtropical climate environment, it prefers water with a pH between 6.5 and 8, a hardness of up to 30 dGH, and a temperature between 20 to 25 °C (68 to 77 °F). In winter, some populations migrate to warmer waters. In the wild, the species' diet consists mostly of crustaceans, annelids, arthropods, and the larvae of these groups, including both aquatic insects such as water beetles, and terrestrial invertebrates such as flies and arachnids that land on or fall into the water. It will also supplement its diet with algae or aquatic vegetation. In captivity, it is largely omnivorous, and typically thrives on a wide variety of foods including frozen or thawed or live cultured blackworms, bloodworms, brine shrimp, daphnia, mysis shrimp, and other commercially available fish foods. A. mexicanus has previously been treated as a subspecies of A. fasciatus, though this classification is not widely accepted. Additionally, the hypogean blind cave form is sometimes classified as a separate species named A. jordani, but this classification directly contradicts phylogenetic evidence.
